Ancient people were perplexed by the motion of planets. Unlike stars which remained in stable positions relative to one another, planets apparently wandered aimlessly across the heavens. In 2006 Victoria Hutchins did a science fair project in which she used a telescope to make observations of the phases of the planet Venus and the moon to prove that the solar system matched a heliocentric model --- planets revolve around the sun --- rather than a geocentric model --- revolve around the Earth.
A solar system model is a science fair classic. However, without a hypothesis and subsequent investigation a model is nothing more than an art project. One idea is to compare the relative size of each planet with its gravity. Another idea is to create a scale model of the solar system outdoors. You will need a camera to document this project as it will not fit in most buildings. Using a sun the size of an orange and a scale of 10 million miles per yard Mercury will be about three-and-a-half yards from the sun while Neptune will be 280 yards away. Want to add the dwarf planet Pluto? At its most distant Pluto will be 453 yards from the orange.
The two most prominent celestial bodies in the solar system from Earth's perspective are the sun and moon. A simple but informative project is to measure the diameter of the moon and the sun. Cut a hole in a small piece of cardboard and cover with tin foil. Poke a tiny hole in the foil. Hold the cardboard where sunlight can shine through onto a piece of white paper. Measure the diameter of the image and the distance the viewer is from the paper. Divide the diameter by the distance and multiply by 93 million to get an approximate diameter of the sun. Measure the moon when it is full and multiply by 283,000.
The eight planets of the solar system are stretched out over unimaginable distances. How does the distance from the sun impact each planet? Janice VanCleave designed a project to demonstrate the effect using transparent tape, graph paper, ruler, flashlight, felt-tipped pen and a helper. Shine a flashlight on the graph paper at a distance of a 1/2 inch. Moving the flashlight away from the wall demonstrates how solar energy spreads out as it moves into the outer solar system. Trace drawings of your findings to use in a science fair project.